Příplatek není tak vysoký jako měsíční paušál.

Není is the 3rd person singular present tense negative of být (to be).

• je = is
• není = is not

So:

• Příplatek je vysoký. = The surcharge is high.
• Příplatek není tak vysoký jako měsíční paušál. = The surcharge is not as high as the monthly flat fee.

The subject is příplatek, which is singular, so není must also be singular.

Tak ... jako is a very common Czech comparison pattern meaning as ... as.

In this sentence:

• tak vysoký jako = as high as

So the full structure is:

• X není tak vysoký jako Y
  = X is not as high as Y

This is one of the standard ways to make an equal comparison in Czech.

A close equivalent is sometimes stejně ... jako, but tak ... jako is especially common with adjectives:

• není tak drahý jako... = is not as expensive as...
• není tak vysoký jako... = is not as high as...

Czech often uses vysoký (high) and nízký (low) for amounts, prices, fees, salaries, charges, and similar things.

So:

• vysoký poplatek = a high fee
• vysoká cena = a high price
• vysoký plat = a high salary

That means Příplatek není tak vysoký... sounds natural in Czech for talking about the amount of the surcharge.

English sometimes prefers:

• not as high
• not as large
• or even not as expensive, depending on context

But in Czech, vysoký is the normal choice here.

Because they are adjectives agreeing with masculine singular nouns.

• příplatek is masculine inanimate singular
• paušál is also masculine inanimate singular

So the adjectives must match them:

• tak vysoký příplatek / Příplatek je vysoký
• měsíční paušál

In Czech, adjectives change their endings to match the noun’s:

• gender
• number
• case

Here, both nouns are masculine singular, so -ý is the expected nominative ending.

Here it is in the nominative.

That is because in this kind of comparison, jako often links two things in the same grammatical role. The sentence compares:

• příplatek
  with
• měsíční paušál

Both are being treated as the compared items, so měsíční paušál stays in the same case as příplatek, which is nominative.

So:

• Příplatek = nominative subject
• měsíční paušál = nominative in the comparison

This is normal in Czech comparison structures like tak ... jako.

Příplatek usually means an extra charge, surcharge, supplement, or additional fee.

It is built from:

• při- / historically related to the idea of added
• platek from the same family as paying

In practical use, it often means you pay something on top of a basic price or service.

Examples:

• příplatek za zavazadlo = extra charge for luggage
• příplatek za expresní doručení = surcharge for express delivery

So in your sentence, it suggests some extra fee is being compared with a monthly fixed fee.

Paušál usually refers to a flat-rate charge, fixed monthly fee, or a plan/tariff depending on context.

With měsíční in front of it, měsíční paušál most naturally means:

• monthly flat fee
• monthly fixed charge
• sometimes monthly plan fee

This word is especially common in contexts like:

• phone plans
• subscriptions
• utilities
• service contracts

So it suggests a regular recurring amount, unlike příplatek, which is an extra payment.

Normally, no—not in standard Czech.

The natural pattern is:

• tak + adjective + jako

So:

• není tak vysoký jako... = correct
• není vysoký jako... = not natural / generally incorrect in standard Czech for this meaning

If you want to express comparison of equality or inequality, tak is an important part of the structure.

Yes. A common alternative is to use the comparative:

• Příplatek není vyšší než měsíční paušál.

This means:

• The surcharge is not higher than the monthly flat fee.

Difference in feel:

• není tak vysoký jako = is not as high as
• není vyšší než = is not higher than

These are very close in meaning, but the grammar is different:

• tak vysoký jako = equality-style comparison
• vyšší než = comparative comparison

Both are natural Czech.

Yes, Czech word order is fairly flexible, but the neutral version is:

• Příplatek není tak vysoký jako měsíční paušál.

You can move things around for emphasis, for example:

• Tak vysoký jako měsíční paušál příplatek není.

This is still understandable, but it sounds more marked or emphatic.

For learners, the safest choice is to keep the basic order:

• subject + není + tak + adjective + jako + comparison

A rough pronunciation guide:

• Příplatek ≈ PZHEE-pla-tek
• není ≈ NEH-nee
• tak ≈ tahk
• vysoký ≈ vih-soh-KEE
• jako ≈ ya-koh
• měsíční ≈ mnyeh-SEECH-nyee
• paušál ≈ pow-SHAHL

A few useful pronunciation notes:

• ř in Příplatek is a special Czech sound with no exact English equivalent.
• ě in měsíční softens the consonant before it.
• š is like sh in shoe.
• Stress in Czech is usually on the first syllable of each word:
  • PŘÍplatek
  • NEní
  • VYsoký
  • MĚsíční
  • PAušál

Yes. Měsíční means monthly.

It comes from měsíc, which can mean:

• month
• or moon, depending on context

As an adjective:

• měsíční platba = monthly payment
• měsíční poplatek = monthly fee
• měsíční paušál = monthly flat fee / monthly plan fee

So it simply describes the type of paušál.

The subject is Příplatek.

You can tell because:

• it is in the nominative
• the verb není is singular and agrees with it
• the sentence is saying something about the surcharge

So the structure is:

• Příplatek = subject
• není = verb
• tak vysoký = predicate adjective phrase
• jako měsíční paušál = comparison part

In simple terms: the sentence is describing the surcharge and comparing it to the monthly fee.